There has been some threads, inquiries and one’s self answers about propeller pitches on Ribnet. Found this nice information about this misunderstood issue, here’s the condensed article important facts to be aware of :

Best all around performance is achieved when wide open throttle (WOT) engine operation occurs near the top (but within) the wide open throttle RPM operating range design by the manufacturer for that specific engine. Improperly propping an engine can not only reduce best performance, but in fact damage and ruin the engine.

An engine that does not reach the rated RPM at Wot is in an over propped condition resulting in lugging. This high torque operation puts a tremendous load on the pistons, crankshaft and bearings. The engine runs much hotter and may overheat from having over advanced spark timming for the reduced amount of fuel entering the engine.

The mechanical strain on an over propped marine engine is like starting an automobile in third gear from a dead stop at the bottom of a hill. The resulting severe strain can lead to detonation, piston seizure and severe engine damage.

On the other hand, an engine that revs past the recommended RPM at Wot will have higher than normal wear and can also be damaged by fatigued parts breaking and passing through the engine. This is why is so critical to be sure your engine is correct propped for your boat/engine combination and the type of boating you want to do. To make this selection propeller charts are published as general guidelines for general applications, they are not intended however to be the absolute recommendation as boats and operation conditions vary . Remember the best propeller selections for your particular boating needs can be determined only by experimentation.

You should have more than one prop if you use your boat/engine for more than one type of activity, such as cruising, fishing, skiing, you may well need different props for the best performance in each type of activity. In any event you should keep on board at all times, along with a nut wrench , pliers, spare nut and tab washer. Propeller lines normally are designed so that the next size pitch will change rpm by 300 to 500. So if the engine rpm falls too low on your first prop selection, try a lower pitched prop to bring the rpm. Higher pitched props reduces the engine rpm, lower pitched props increases engine rpm.

Pitch Change Calculations : Just follow these steps : A tacho will be a must.

Fine Tunning Adjustment: For every 1” of pitch change, the effect will be aprox 200 rpm. Knowing this, take the difference in the above example at 800 and divided by 200. The result is 4”. So, the next prop to try will be 4” in pitch less than the prop which was first used. You should now either have the right prop or be only one size off.

Propeller lines normally are designed so that the next size pitch will change rpm by 300 to 500. So if the engine rpm falls too low on your first prop selection, try a lower pitched prop to bring the rpm higher Higher pitched props reduces the engine rpm, lower pitched props increases engine rpm.

Pitch Change Calculations : Just follow these steps : A tacho will be a must.

Fine Tunning Adjustment: For every 1” of pitch change, the effect will be aprox 200 rpm. Knowing this, take the difference in the above example at 800 and divided by 200. The result is 4”. So, the next prop to try will be 4” in pitch less than the prop which was first used. You should now either have the right prop or be only one size off.

Hope this article helps solve many pitch prop technical doubts…

Happy Boating
Locozodiac

The previous article had a erroneous data reffered to the top operating range, read 6000 rpm instead of the max 5600 rpm used on this example, this might have lead to confussion, sorry.

Aluminum to stainless (identical in all other ways) should see a 100 to 200 rpm difference (drop) due to the stiffer material. Hardly ever applicable directly, as things like cupping and blade shape play a large role in the actual rpm change as well. Nos's rule of thumb seems reasonable.

A change in prop diameter of 1/4" should give see a change of about 100 rpm (smaller diameter will increase rpm)

A pitch change of 1" should see about a 200 to 250 rpm difference (Larger pitch numbers dropping rpm.)

Of course, these are guidelines, not "rules", as every boat and every prop reacts differently.

I personally feel that WOT prop selection should be done with a "normal" load (as in, how the boat will be run most of the time.), rather than with the light load that Loco suggested. The slight over-rev when running light will be offset by having the capability of revving to max rpms most of the time.

"I personally feel that WOT prop selection should be done with a "normal" load (as in, how the boat will be run most of the time.), rather than with the light load that Loco suggested. The slight over-rev when running light will be offset by having the capability of revving to max rpms most of the time".

This is not my suggestion, it's the article's, the reason of wotting light is because if you ever need to full wot this way (a very rare occasion) you'll be at the max engine rpm that the manufacturer recommends. A sort or over rev potection when running light. Can be applied for racing.

On the other hand many small to medium users (tiller mostly) don't use a tacho ever, generally use a mid pitched propeller that most of the time is standard factory delivered and perhaps will never wot their boat/engine to obtain the best performance that can be achieved. I've seen engines with prop changed, thinking that will perform better, but running under or over rev, a short path to premature engine disaster. Most customers don't know anything about correct prop tacho wotting, unless a well trainned dealer offers them this precise info including test props to top their engine/boat performance.

Have wotted my engine this way and can full throttle knowing that my engine is at the max rev safely. Just as a experiment and to take the nail out, wot light & normal and see how much rpm & speed difference between both. Would be nice to write any outboard factory and ask them which of the stated two possibilities will be the correct or applies better. Anyway...

As I understand it, if 1 revolution of prop produces forward travel of 22" then this is the pitch of the prop...22"
Therefore, why do you say that equivalent of 23" alloy is a 22" stainless? Surely a 22" is a 22" and a 23" is a 23" ?

However the entire force pushing your boat along is supported by the blades. The pressure is quite large, and so the stiffness of the material comes into play. The prop blades will deform under the pressure, altering the pitch, the ally ones more than the stainless. The mechanics of it will be down to the design of the blades, the force they are trying to withstand etc etc.

As for the odd / even thing it certainly doesnlt apply on small Suzis - My 25 had a choice of 11,12,13,14 & 15"...... Maybe that rule holds higer up the HP scale?

As I understand it, if 1 revolution of prop produces forward travel of 22" then this is the pitch of the prop...22"
Therefore, why do you say that equivalent of 23" alloy is a 22" stainless? Surely a 22" is a 22" and a 23" is a 23" ?

As 9D280 says, deformation comes into play. Also the thinner the blades (ie stainless) the less slip you get.Therefore a 21" or 22" stainless prop will produce as much forward movement as a 23" ally one.